You can only broach a through hole, not a blind hole as the broach must be pulled through the workpiece. Originally they would have used a slotting type head which would have shaved it out a thou at a time. The Italian Gun Makers Piotti have such a machine and you can see it in use on their website although it is cutting the slot for the lumps. It is the same principal however. Aaron is correct, these days it would be done with EDM.

http://www.piotti.com/en/lavorazioni/la-meccanica/

Waidmannsheil.